This 5 year competing application describes an opportunity to explore the long term outcomes of thymus transplantation in detail, with particular focus on the role of thymic activity and T cell function. Over the last 15 years, our laboratory has established that thymus transplantation is a successful therapeutic strategy for infants born with complete DiGeorge anomaly (cDGA) who are athymic and thus have a fatal immunodeficiency. Thymus transplantation results in host T cell reconstitution and survival of approximately 73% of subjects. We now will address unanswered questions about the basic biology of thymus transplantation in our long term survivors. Questions include the following, 1) why do T cell numbers remain in the 10th percentile after cDGA subjects develop circulating T cells and T cell function? How long does the thymus function after transplantation? 2) Do the peripheral T cells undergo premature senescence and die at higher rates than normal from apoptosis? 3) How are host T cells restricted to host HLA after development in an unmatched donor thymus? What recipient cells are present in the donor thymus after transplantation that can effect positive selection? To address these questions, we will pursue three aims. The first specific aim will assess the etiology of low T cell numbers after transplantation. Numbers of recent thymic emigrants (RTE) will be quantified by flow cytometry. Thymic output of RTE will be quantified by in vivo deuterated water loading in cDGA subjects. The intrathymic proliferation of thymocytes will be quantified by determination of the signal joint to ? T cell receptor rearrangement excision circle (TREC) ratios. These data will be evaluated to determine if thymic function decreases with time after transplantation. The RTE and TREC data from cDGA subjects will be compared to data from partial DGA and healthy children to assess relative thymic output in cDGA. The second specific aim will address the survival of peripheral T cells. Quantification of telomere length in naove CD4 cells will show if there is premature senescence of these cells. Studies of apoptosis will address whether peripheral apoptosis contributes to the low T cell numbers found in subjects with cDGA. The third specific aim will assess the binding of T cell receptors to HLA by tetramer experiments. The binding to class I and class II tetramers will be used to evaluate restriction to host and donor HLA. Affinity of binding to HLA Class I tetramers will be assessed for cDGA subjects with and without matching to the thymus donor and will be compared to the binding of subjects with partial DiGeorge anomaly. By comparing the outcomes of the experiments from cDGA subjects to those with partial DGA, who have lymphopenia, and to those of healthy age matched children, we can determine if thymus transplantation across an HLA barrier contributes to defects in T cell numbers after transplantation.